Is CVE-2026-86205 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-86205 being exploited in the wild.

How severe is CVE-2026-86205?

Medium severity, with a CVSS score of 5.4, published 6 September 2026.

Which products and versions are affected?

h3 (npm) SEMVER:introduced=2.0.1-rc.17,fixed=2.0.1-rc.18.

Is there a fix for CVE-2026-86205?

Yes. h3 (npm): upgrade to 2.0.1-rc.18 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-86205 attacked?

It is attacked over the network.

Am I affected by CVE-2026-86205?

Check whether you run h3 (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-86205: H3 open redirect via protocol-relative path in redirectBack()

CVE-2026-86205 · Severity: medium · CVSS 5.4 · Published 2026-09-06

H3 is a popular HTTP framework used to build web applications and APIs. The redirectBack() function, which safely redirects users back to their referrer after form submission, contains a flaw that allows attackers to redirect users to external phishing sites by crafting malicious URLs. This can lead to credential theft and compromise of user accounts through phishing attacks.

Technologies: h3 (npm). Vendors: npm.

Quick answers

Is CVE-2026-86205 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-86205 being exploited in the wild.
How severe is CVE-2026-86205?: Medium severity, with a CVSS score of 5.4, published 6 September 2026.
Which products and versions are affected?: h3 (npm) SEMVER:introduced=2.0.1-rc.17,fixed=2.0.1-rc.18.
Is there a fix for CVE-2026-86205?: Yes. h3 (npm): upgrade to 2.0.1-rc.18 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-86205 attacked?: It is attacked over the network.
Am I affected by CVE-2026-86205?: Check whether you run h3 (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

h3 (npm): upgrade to 2.0.1-rc.18 or later
```
npm install h3@2.0.1-rc.18
```

Technical details

The vulnerability is an open redirect (CWE-601) in the redirectBack() utility function located in src/utils/response.ts. The function validates that a Referer header shares the same origin as the current request before using its pathname for the Location header. However, it fails to sanitize protocol-relative paths (starting with //). An attacker can craft a URL like http://target.com//evil.com/path that passes the origin check (since the origin is target.com) but extracts a pathname of //evil.com/path, which browsers interpret as a protocol-relative redirect to an external domain. The vulnerability requires user interaction (the user must follow an attacker-crafted link and then trigger a redirectBack() call), is network-accessible, and requires no authentication. A fix is available in version 2.0.1-rc.18 and later, which sanitizes the pathname to prevent protocol-relative paths.

Affected products

H3 H3 2.0.1-rc.17

Identifiers

CVE-2026-86205
GHSA-fp4x-ggrf-wmc6

Timeline

2026-03-23: disclosed
2026-03-23: patched: Fixed in version 2.0.1-rc.18

References

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